Single crystals of lithium niobate (hereinafter LiNbO.sub.3) in general are grown by Czochralski method in air or under a controlled gas mixture comprising 20% by volume or more of oxygen and nitrogen. These LiNbO.sub.3 single crystals are advantageous in that they show large electrooptical effects, photoacoustic effects, and non-linear optical effects. Accordingly, extensive and intensive efforts have gone into development of optical devices using the LiNbO.sub.3. Among them, most extensively studied are optical waveguides based on titanium-diffused crystal substrates. The LiNbO.sub.3 crystals, however, suffer change in refractive index when subjected to a strong incident beam, i.e., the so-called optical damage occurs. The optical waveguides mentioned above comprising titanium-diffused crystals are also subject to such optical damage. Optical damage is undesirable to waveguides because, when the damage occurs, the refractive index of the waveguide portion approaches that of the waveguide sheath, and consequently causes the unfavorable light leakage. Concerning the mechanism of the occurrence of this optical damage, on the other hand, it is qualitatively explained as follows. That is, the carrier electrons excited from the impurity level to the conductive level by the incident beam transfer along the c axis from its negative side to the positive side for a certain distance, and are then trapped in the defects such as impurities and vacancies. As a consequence, a spatial electric field, Esc, is developed in the crystal. Thus, there is induced a change in refractive index ascribed to the above-developed spatial electric field, which corresponds to the optical damage.
The change in refractive index, .DELTA.n, of the optical waveguide induced by the optical damage explained above is expressed by EQU .DELTA.n=-1/2n.sup.2 .multidot.r.multidot.Esc (1)
where, n represents the refractive index, r represents the electrooptical constant[cm/V], and Esc represents the spatial electric field[V/cm].
There is proposed, according to the aforementioned mechanism, two methods to reduce the optical damage. One is to add magnesium oxide to the single crystal LiNbO.sub.3, and the other is to reduce the amount of the impurities incorporated into the crystal.
The former method comprises growing a crystal by Czochralski method from a LiNbO.sub.3 melt added therein 5% by molar of MgO. It is reported that the resulting single crystal is improved in resistance against optical damage by about two orders of magnitude as compared with one free from additives. When an optical waveguide is produced from this single crystal diffused thereon titanium, the resulting product still suffers optical damage in the waveguide portion.
The latter method comprises reducing the content of impurities, particularly that of the transition elements such as iron, because those transition elements are considered most responsible for the optical damage. The crystals obtained in this method are, however, yet to be improved in purity.
As set forth above, there is no means up to the present to reduce the optical damage of optical waveguides produced by diffusing titanium on LiNbO.sub.3 single crystal substrates.